Optical encoders use optical signals to detect mechanical positions and motions in various types of systems. The mechanical positions and motions detected by optical encoders can be related to linear or rotational displacements of moveable components, such as shafts of motors. There are two common types of optical encoders, absolute position encoders and incremental encoders. Both types of optical encoders can determine mechanical positions and motions. The absolute position encoders can determine the exact mechanical position at any moment of operation, even at power-up. The incremental encoders, on the other hand, lose the existing position data at power-down, and cannot determine the exact mechanical position at power-up. However, the incremental encoders are less expensive and require less processing power than the absolute position encoders. Thus, the incremental encoders enjoy a greater market share than the absolute position encoders.
As shown in FIG. 1, a conventional incremental optical encoder 100 typically includes a light source 102, a mask plate 104, an opaque encoder member 106, a pair of optical detectors 108A and 108B and a processor 110. The encoder member 106 includes a first track 112A of small openings 114A and a second track 112B of small openings 114B. The encoder member 106, which is shown in FIG. 1 as a rotary disk, is positioned between the light source 102 and the two optical detectors 108A and 108B.
In operation, the light source 102 emits a beam of light through the mask plate 104, which shapes the beam of light into an elongate beam of light along the Y-direction. The elongate beam of light then strikes the tracks 112A and 112B. As the encoder member 106 is rotated, some of the beam of light is transmitted through the small openings 114A on the track 112A and received by the photodetector 108A, while some of the beam of light is transmitted through the small openings 114B on the other track 112B and received by the photodetector 108B. The photodetectors 108A and 108B generate electrical signals in response to the received light. As shown in FIG. 1, the openings 114A on the track 112A and the openings 114B on the track 112B are offset from each other so that the optical detectors 108A and 108B generate quadrature signals when the encoder member 106 is rotated. The quadrature signals are transmitted to the processor 110, which can process the signals to determine the speed, direction and/or position of the encoder member 106.
A concern with the conventional incremental encoder 100 is that the rotary disk 106 with the openings 114A and 114B on the tracks 112A and 112B is relatively expensive to manufacture, which is reflected in the overall cost of the encoder. Another concern is that the encoder 100 is limited with respect to detecting small positional changes of the rotary disk 106, which depends on the spacing of the openings 114A and 114B on the tracks 112A and 112B.
In view of these concerns, there is a need for a cost-effective incremental optical encoder with greater sensitivity with respect to detection of small positional changes.